Pyrolytic boron nitride crucible and method for producing the same

ABSTRACT

A pyrolytic boron nitride crucible is provided. The crucible contains at least five wall layers consisting of first and second wall layers. The thickness of each of the first wall layers is 5 to 100 microns and the thickness of each of the second wall layers is 1/50 to 1/1 of that of the first wall layer. The total thickness of the entire wall layers is 0.5 to 3 mm. The first wall layer is bonded to and laminated alternately with the second wall layer.

This application is a continuation of application Ser. No. 216,589,filed July 7, 1988, abandoned, which in turn is a continuation of Ser.No. 866,823, filed May 22, 1986, now U.S. Pat. No. 4,773,852.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pyrolytic boron nitride crucible anda method for producing the same, and particularly to a pyrolytic boronnitride crucible used in the Molecular Beam Epitaxy method or the LiquidEncapsulated Czochralski method for melting therein a metal or acompound and a method for producing such a crucible.

2. Related Art Statement

Pyrolytic boron nitride is high grade boron nitride of high purity andused for wide applications including the production of compoundsemiconductors and special alloys. Particularly, in the production of acompound semiconductor such as GaAs the excellent anticorrosive propertyand the high purity of the pyrolytic boron nitride are effectivelyutilized for the growth of a single crystal of a compound semiconductorcontaining little impurities and superior in electrical properties. Forexample, in the process for growing a GaAs single crystal, the pyrolyticboron nitride is used for a material for a crucible in the LiquidEncapsulated Czochralski method and is also used for a material for aboat in the horizontal Bridgeman technique. Moreover, the pyrolyticboron nitride is almost exclusively used for a material for a cruciblein which a metal is melted in the Molecular Beam Epitaxy method, whichis a method for growing epitaxitially a mixed crystalline compoundsemiconductor, such as Ga_(1-x) Al_(x) As on a wafer made of a singlecrystal of GaAs.

Such pyrolytic boron nitride has been produced through the so-calledchemical vapor deposition method, as disclosed by U.S. Pat. No.3,152,006, wherein a boron halide, such as boron trichloride (BCl₃) andammonia are used as gaseous starting materials to deposite boron nitrideat a temperature of from 1450° to 2300° C. and at a pressure of not morethan 50 Torr on the surface of an appropriate substrate. Then, thedeposited pyrolytic boron nitride is separated or released from thesubstrate to obtain an article made of self-standing pyrolytic boronnitride. The thus obtained pyrolytic boron nitride has a structurewherein the C-planes of the hexagonal crystal lattices are oriented inthe direction perpendicular to the growth direction of the depositedwall layer, and thus the properties thereof are exceedingly anisotropic.The pyrolytic boron nitride has a high mechanical strength in thedirection parallel to the surface of the wall layer, but the mechanicalstrength thereof along the crystal growth direction is not so high thatthe formed wall layer tends to be exfoliated along the growth ordeposition direction. Such a tendency of exfoliation along thedeposition direction is a main cause for reducing the lifetime of acrucible made of pyrolytic boron nitride when used repeatedly. Problemsinvolved in the conventional crucibles used in the Liquid EncapsulatedCzochralski method and the Molecular Beam Epitaxy method will bedescribed in detail hereinbelow.

A sinble crystal rod of a compound semiconductor, such as GaAs or InP,has been predominantly produced through the Liquid EncapsulatedCzochralski method comprising the steps of melting the aforementionedcompound or materials for the compound in a crucible made of pyrolyticboron nitride, covering the upper surface of the mass with molten B₂ O₃liquid of high purity to be encapsulated, dipping a seed crystal of thecompound in the molten mass of the compound, and then pulling up theseed crystal slowly from the molten mass to form a single crystal rod ofthe compound. After the completion of pulling up the single crystal, thecrucible must be cooled to room temperature and the cooled andsolidified B₂ O₃, which has served as the encapsulator and is nowadhering on the interior periphery of the crucible, must be removedprior to repeated use. However, in the operation of removing thesolidified B₂ O₃, portions of the pyrolytic boron nitride layer arefrequently peeled off from the interior surface of the crucible togetherwith the B₂ O₃. It is an extremely rare case where peeling of thepyrolytic boron nitride layer occurs uniformly over the interior wall ofthe crucible, and it frequently occurs that a portion of the interiorwall of the crucible is peeled off and the peeled fragment has a randomthickness. In the most serious case, such a local peeling extends to theexterior peripheral surface of the crucible to result in breakdown ofthe crucible. Even when the defect caused by peeling is not so great indepth, the interfaces between the adjacent boron nitride wall layerscontact with the molten mass so that trace impurities contained in themolten mass are collected in the interfaces. Such locally collected orconcentrated impurities cause disadvantages in that the impurities arereleased in the molten mass at a later operation cycle or a furtherpeeling propagates from such a location. In order to exclude suchdisadvantages, it is required to remove the interior wall layer until asmooth periphery is formed at the depth of the deepest defect caused bypeeling. However, in this operation of forming a smooth interiorperiphery, a deeper defect is apt to be formed by peeling. For thesereasons, the wall of the crucible becomes thinner as it is repeatedlyused for growing therein a semiconductor crystal, although it is notbroken due to a single peeling. As a result, the lifetime of theconventional crucible has been used up after several to ten time usagesthereof.

When the pyrolytic boron nitride is used as a material for a crucibleused in the Molecular Beam Epitaxy method and a metal having goodwettability with the pyrolytic boron nitride, such as aluminum, ismelted therein, the crucible is cooled at the time of stopping theMolecular Beam Epitaxy system, whereupon a severe stress is applied onthe crucible due to tremendous differences in thermal expansioncoefficients between the crucible and the metal contained therein. Sincethe cooled and solidified metal adheres firmly to the interior wallsurface of the crucible, because of good wettability of the metal withthe crucible, the crucible is often broken by the stress.

In order to solve the aforementioned problems, a multi-walled cruciblehaving a thick outer wall layer for providing integrity of the wholestructure, an intermediate wall layer and an innermost wall layer, whichlayers have thin thicknesses and are weakly bonded to the outer layer,has been proposed, for example by U.S. Pat. Nos. 3,986,822 and4,058,579, and commercially sold. Although such a multi-walled crucibleis successfully used as a crucible for melting a metal in the MolecularBeam Epitaxy method, the metal contained therein oozes toward theoutside of the interior wall layer to make further use thereofimpossible once the innermost layer has been damaged, as has beenclearly described in the technical information by Union Carbide Co.,U.S.A. The conventional multi-walled crucibles have been produced by acumbersome and time-consuming process including the steps of depositingone wall layer, interrupting the deposition reaction by cooling, andthen re-starting the deposition reaction by raising the temperature tothe deposition temperature.

OBJECTS AND SUMMARY OF THE INVENTION

A primary object of this invention is to provide a pyrolytic boronnitride crucible which can be used repeatedly for a number of times andhas an extremely long lifetime, and to provide a method for producingthe same.

Another object of this invention is to provide a pyrolytic boron nitridecrucible which is particularly suited for melting therein a metal or acompound in the Molecular Beam Epitaxy method and in the LiquidEncapsulated Czochralski method, and to provide a method for producingthe same.

A further object of this invention is to provide a pyrolytic boronnitride crucible which may be produced at a high production efficiencyand at a low cost through a method which does not include the step ofinterrupting the deposition operation, and to provide a method forproducing the same.

The above and other objects of this invention will become apparent fromthe following detailed description of the invention.

According to the invention, there is provided a pyrolytic boron nitridecrucible having a multi-walled structure comprising at least five walllayers, the thickness of each of first wall layers being 5 to 100microns, the thickness of each of second wall layers being 1/50 to 1/1of that of the first wall layer, the total thickness of the entire walllayers being 0.5 to 3 mm, and the first wall layer being bonded to andlaminated alternately with the second wall layer, whereby the peeling ofone wall layer is prevented from affecting the other wall layers so thatthe peeling of one wall layer is not propagated into the adjacent layer.

According to a further aspect of this invention, there is provided animprovement in the method for producing a pyrolytic boron nitridecrucible having a multi-walled structure comprising at least five walllayers bonded to and laminated with alternately with one another, thethickness of each wall layer being 5 to 100 microns, and the totalthickness of the entire wall layers being 0.5 to 3 mm through repeatedchemical vapor deposition steps using a boron halide gas and an ammoniagas as the starting materials, the improvement which comprises the stepof forming a first wall layer at a deposition rate of 10 to 150 micronsper hour, and the step of forming a second wall layer at a depositionrate of 1.5 to 5 times that of the step for forming the first walllayer, the step of forming the first layer and the step of forming thesecond layer being repeated alternately.

According to a still further aspect of the invention, there is providedan improvement in the method for producing a pyrolytic boron nitridecrucible having a multi-walled structure comprising at least five walllayers, the thickness of each of first wall layers being 20 to 100microns, the thickness of each of second wall layers being 1/50 to 1/2of that of the first wall layer, the total thickness of the entire walllayers being 0.5 to 3 mm, and the first wall layer being bonded to andlaminated alternately with the second wall layer through repeatedchemical vapor deposition steps using a boron halide gas and an ammoniagas as the starting materials, the improvement which comprises the stepof forming the first wall layer by maintaining the molar ratio ofammonia to boron halide in the starting gas mixture within the range offrom 2 to 10, and the step of forming the second wall layer bymaintaining the molar ratio of ammonia to boron halide in the startinggas mixture within the range of from not less than 1/3 to less than 2,the step of forming the first layer and the step of forming the secondlayer being repeated alternately.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by reference to the accompanyingdrawing, which is a cross-sectional view showing a pyrolytic boronnitride crucible having a multi-walled structure according to theinvention.

DESCRIPTION OF THE INVENTION

The present invention will now be described more in detail.

The pyrolytic boron nitride crucible having the multi-walled structure,according to this invention, have a plurality of first wall layers, eahhof which should have a thickness of from 5 to 100 microns, preferablyfrom 20 to 100 microns. If the thickness of each first wall layer isless than 5 microns, the individual wall layers are not distinguishablysaparated with each other to lose the merit of the multi-walledstructure. On the contrary, if the thickness of each first wall is morethan 100 microns, spontaneous delamination occurs at the interfaces ofthe adjacent wall layers due to the stress developed in-between the walllayers, leading to disadvantages such that impurities are trapped in theinterfaces of adjacent wall layers.

Each of the second layers should have a thickness of 1/50 to 1/1,preferably 1/50 to 1/2, of that of each adjacent first wall layer. Ifthe thickness of each second wall layer 12 is less than 1/50 of that ofthe first wall layer 11, the merit of the multi-walled structure is notexhibited. On the contrary, if the thickness of each second wall layer12 is more than 1/1 of that of the first wall layer 11, delaminationoccurs at the interfaces between the first and second wall layers 11,12. Total thickness l of the entire wall layers should be within therange of from 0.5 to 3 mm. If the total wall thickness l is less than0.5 mm, the strength of the crucible 10 is insufficient, whereasspontaneous delamination occurs due to the increase of internal stressif the total wall thickness l exceeds 3 mm.

The multi-walled crucible of the invention has a lifetime which isremarkedly longer than those of the conventional pyrolytic boron nitridecrucibles. The crucible of the invention, when used as a crucible in theLiquid Encapsulated Czochralski method, will now be described. Since thecrucible 10 of the invention has a number of pyrolytic boron nitridewall layers 11, 12 bonded, preferably weakly bonded, with each other,even if the inner pyrolytic boron nitride wall layer 11a is peeled offduring the step of removing B₂ O₃ after the completion of growth of thecrystal by the Liquid Encapsulated Czochralski method, peeling isselectively initiated only at the interface between the adjacent walllayers 11a, 12a because of the fact that the bonding force at theinterface of adjacent wall layers 11a, 12a is intentionally designed tobe weaker than the internal portions of each wall layer. In addition,the peeling is propagated only along the interface of adjacent walllayers 11a, 12a, and increase in thickness of the exfoliated fragment,otherwise found in the conventional pyrolytic boron nitride crucible,along with the propagation of peeling does not take place. By theprovision of the first wall layers 11 each having a thickness of from 5to 100 microns, peeling occurs always at the innermost wall layer 11a ofthe crucible 10 and not found at the interstices between the othermultiple pyrolytic boron nitride wall layers, although the reasontherefor has not been made clear. Since the peeling caused at the stepof removing B₂ O₃ takes place always at the innermost wall layer 11a andthe peeling is not propagated into the adjacent layer, the exfoliatedlayer may be removed without affecting the adjacent layer. As a result,the lifetime of the crucible 10 is prolonged remarkably, and thescattering in lifetime of individual crucibles is little.

When the crucible 10 of the invention is used for melting therein ametal in the Molecular Beam Epitaxy method, the innermost pyrolyticboron nitride wall layer 11a adheres firmly with the solidified metal atthe cooling step, thereby being applied with an inward tension stress.However, according to the present invention, since each of the walllayers 11, 12, including the innermost wall layer 11a, is as thin as 100microns at the thickest to have some flexibility and only weakly bondedto the outside wall layer, the innermost layer 11a adhering to the metalis separated slightly from the adjacent wall layer 12a to be shrunkwithout being broken. Even if the innermost wall layer 11a is broken andthe molten metal penetrates in between the adjacent wall layers 12a,there is left a number of pyrolytic boron nitride wall layers 11, 12having similar properties as those of the innermost wall layer 11aaround the periphery of the innermost wall layer. Accordingly, thecrucible 10 of the invention can be used repeatedly even after theinnermost wall layer 11a has been peeled off. The pyrolytic boronnitride crucible 10 of the invention can, thus, be used repeatedly moreand more times as compared with prior art crucibles.

The method of producing the pyrolytic boron nitride crucible, accordingto the invention, will now be described.

Each of the wall layers forming the multi-walled pyrolytic boron nitridecrucible may be deposited by a chemical vapor deposition or chemicalvapor growth technique while using a boron halide gas, such as borontrichloride gas, and an ammonia gas as the starting gaseous materials.It is desirous that the pressure in the chemical vapor deposition stepranges within 0.5 to 5 Torr and the temperature at the step ismaintained within 1850° to 1950° C. If the pressure is less than 0.5Torr, decomposition of pyrolytic boron nitride is activated, whereasapplication of a pressure of higher than 5 Torr is not preferred sincefine particles of boron nitride are formed by the side reaction so thatthe fine particles are introduced in the pyrolytic boron nitride walllayer to impair the uniformity of the resultant structure. If thechemical vapor deposition step is carried out at a temperature of lowerthan 1850° C., the strength of the formed pyrolytic boron nitride islowered so as not to form a crucible withstanding practical use. On thecontrary, if the chemical vapor deposition step is carried out at atemperature of higher than 1950° C., pyrolytic boron nitride reacts withcarbon constituting the substrate and is converted into B₄ C whichinduces a disadvantageous result in that the content of impurity carbonis increased in the pyrolytic boron nitride. The most preferred pressureranges from 0.75 to 3 Torr, and the most preferred temperature range isfrom 1900° to 1940° C.

In a first embodiment of the method of producing a boron nitridecrucible having the multi-walled structure with the wall layers beingweakly bonded, according to the invention, each of the layers having athickness of 5 to 100 microns and bonded with one another to form thepyrolytic boron nitride crucible having at least five wall layers shouldbe deposited at a deposition rate (or layer growing rate) which isdifferent from that for the deposition of the adjacent layer. For thispurpose, the first wall layer is deposited at a deposition rate of 10 to150 microns per hour, and the second wall layer adjacent to the firstwall layer is deposited at a deposition rate of 1.5 to 5 times that forthe deposition of the first wall layer, the step for depositing thefirst wall layer and the step for depositing the second layer beingrepeated alternately.

The deposition rate for depositing the second wall layer is set to 1.5to 5 times as high as that for depositing the first layer for thefollowing reasons. If the deposition rate for depositing the second walllayer is less than 1.5 times that for depositing the first layer, thebonding force between the adjacent wall layers is not significantlydifferentiated from the bonding strength internally of a continuous walllayer which has been formed at a constant deposition rate, thus failingto attain the object of the invention for the provision of amulti-walled structure. If the difference in deposition rate is greaterthan 5 times, the bonding force between the adjacent wall layers becomestoo low to cause spontaneous delamination at the interfaces of the walllayers in the crucible. It is desirous that the deposition rate of awall layer be not more than 750 microns per hour at the greatest, sincethe mechanical strength of a pyrolytic boron nitride wall layer islowered extremely when the deposition rate is set to a level of morethan 750 microns per hour. Accordingly, the deposition rate fordepositing the first wall layer, which should be within the range of1/1.5 to 1/5 of that for depositing the second layer, may be not morethan 150 microns per hour as a natural consequence. It is preferred thatthe first wall layer is deposited at a deposition rate of not less than10 microns per hour, in order to obviate unnecessary prolongation of thetime required for forming one crucible. Preferable conditions forproducing a crucible having a satisfactory quality at an acceptableproduction efficiency are that the first wall layer is formed at adeposition rate of from 50 to 150 microns per hour and that the secondwall layer is formed at a deposition rate of from 75 to 250 microns perhour.

Although the first and second wall layers should be depositedalternately one after another for producing a crucible of the invention,interruption of operation for shifting the deposition condition fromthat for the first wall layer to that for the second wall layer is notnecessary, and the shift or change of deposition condition may beeffected continuously without the need of standstill time. Accordingly,the crucible of the invention can be produced within a timesubstantially the same as that required for the production of aconventional single wall crucible.

In order to shift the deposition rate at the steps of forming,respectively, the first and second wall layers, various parametersaffecting the deposition rate in the ordinary chemical vapor depositionmethod may be changed at certain time intervals. For example, theconcentrations of the starting gases introduced in the chemical vapordeposition reaction chamber or the flow rates of the gases flowing overthe substrate may be changed by changing the compositions of the gases,or by varying the flow rate of the gases or the pressure in the reactionchamber at certain intervals.

In a second embodiment of the method of producing a boron nitridecrucible having the multi-walled structure, according to the invention,wherein each of the first wall layers has a thickness of 20 to 100microns and each of the second wall layers has a thickness of 1/50 to1/2 of that of the first layer, each of the first wall layers beingweakly bonded to and laminated alternately with each of the second walllayers, the molar ratio of ammonia to boron halide for depositing eachfirst wall layer is differentiated from that for depositing each secondwall layer. In this second embodiment of the method of the invention,each of the first wall layers providing the main structural integrity ofthe crucible should be deposited while maintaining the molar ratio ofammonia to boron halide within the range of from 2 to 10. If the molarratio of ammonia to the boron halide is less than 2, the formedpyrolytic boron nitride is not sufficiently flexible and is easilybroken. If the molar ratio of ammonia to boron halide is more than 10, alarge amount of by-product ammonium chloride is formed with attendantproblems which pose difficulty in operation. Each of the second walllayers (intermediate wall layers) which serve to bond the adjacent firstwall layers should be deposited while maintaining the molar ratio ofammonia to boron halide within the range of not less than 1/3 and lessthan 2. If the molar ratio is more than 2, the bonding force between thefirst and second wall layers becomes too strong to cause disadvantageouspeeling of plural wall layers. If the molar ratio is less than 1/3,pyrolytic boron nitride is not formed to deteriorate the anticorrosiveproperty of the formed second wall layer.

A pyrolytic boron nitride crucible having a good quality may beproduced, for example, by a method comprising the first step ofdepositing a first wall layer using a composition containing ammonia andboron halide in a molar ratio of 2.5 to 4.5 to have a thickness of from25 to 50 microns and the second step of depositing a second wall layerusing a composition containing ammonia and boron halide in a molar ratioof 1 to 1.5 to have a thickness of from 5 to 10 microns, the first andsecond steps being repeated alternately until the total wall thicknessreaches 0.9 to 1.5 mm. Since it is not necessary to interrupt thedeposition operation at the time of shifting the condition from that forthe deposition of the first wall layer to that for the deposition of thesecond wall layer, and vice versa, a crucible can be produced withoutany unnecessary prolongation of the time required for the production.

EXAMPLES OF THE INVENTION

The present invention will be described more specifically with referenceto some Examples and Comparative Examples.

Examples 1 to 5 and Comparative Examples 1 to 5

Using eight graphite plates each having the dimensions of 50 cm(width)×60 cm (length)×1 cm (thickness), a reaction chamber having anoctagonal cross section was formed on a graphite plate (bottom plate)having a diameter of 20 cm. A 5 cm diameter hole was provided at thecenter of the bottom plate for introducing therethrough gases. Twographite pipes, one having an outer diameter of 5 cm and the otherhaving an outer diameter of 2.5 cm, coated preliminarily with pyrolyticboron nitride were connected coaxially to the hole of the bottom plate.A graphite substrate having a diameter of 5 cm and a length of 6 cm wassuspended in the upper portion of the reaction chamber. The reactionchamber was put into a vacuum furnace heated to a high temperature byresistance heating, and the graphite inner and outer pipes of thestarting gas inlet conduit were connected with stainless steel gasconduits for feeding, respectively, an BCl₃ gas and a NH₃ gas.

The furnace was evacuated to an order of 10⁻³ Torr and heated to 1900°C., and then pyrolytic boron nitride crucibles each having a total wallthickness of 1 mm were produced at a pressure of 1 Torr and under theconditions as set forth in Table 1. The deposition rate was changed byvarying the concentrations of the starting gases. Each of the thusproduced crucibles was subjected to the following lifetime test whichwas conducted to simulate the operations for growing a single crystal bythe Liquid Encapsulated Czochralski method.

50 g of B₂ O₃ was put into an crucible and heated to 1280° C. in N₂atmosphere to melt B₂ O₃, and then cooled to the room temperature. Thecrucible was dipped in methanol and rinsed by subjecting the same toultrasonic wave oscillation for 20 to 40 minutes to remove B₂ O₃adhering to the interior wall surface of the crucible. Portions of theinner peripheral wall of each crucible adhering to shrunk B₂ O₃ werepeeled off upon cooling of the crucible. The aforementioned simulationtest cycle was repeated until the crucible had been broken. The numberof test cycles conducted until each crucible had been broken is shown inTable 1. A commercially available crucible (Comparative Example 5) wassubjected to a similar test.

                                      TABLE 1                                     __________________________________________________________________________    First Wall Layer    Second Wall Layer                                               Deposition Rate                                                                       Thickness                                                                           Deposition Rate                                                                       Thickness                                                                           Lifetime of                                 Run No.                                                                             (micron/hr)                                                                           (micron)                                                                            (micron/hr)                                                                           (micron)                                                                            Crucible                                    __________________________________________________________________________    Example 1                                                                           10      50    15      50    32 cycles                                   Example 2                                                                           100     30    500     30    28 cycles                                   Example 3                                                                           50      100   150     100   24 cycles                                   Example 4                                                                           50      5     150     5     26 cycles                                   Example 5                                                                           50      50    150     50    36 cycles                                   Com. Ex. 1                                                                          160     105   800     105    3 cycles                                   Com. Ex. 2                                                                          50      120   150     120   Delaminated at                                                                the initial                                                                   stage                                       Com. Ex. 3                                                                          50      105   60      105   15 cycles                                   Com. Ex. 4                                                                          50      3     150     4     16 cycles                                   Com. Ex. 5                                                                          Commercially Available Crucible (Single Wall Structure)                                                   15 cycles                                   __________________________________________________________________________

Each of the crucibles of the present invention had a lifetime of morethan 20 cycles, and exhibited a longer life as compared with those ofthe Comparative Examples.

Examples 6 and Comparative Examples 6 and 7

Pyrolytic boron nitride crucibles each having a diameter of 3.49 cm anda height of 4.13 cm were produced. Each of the first wall layers wasformed at a deposition rate of 50 microns/hr to have a thickness of 50microns, and each of the second wall layers was formed at a depositionrate of 150 microns/hr to have a thickness of 50 microns. Ten crucibles(Example 6) were produced under the aforementioned conditions. Tendouble-wall crucibles (Comparative Example 6) each having a 0.4 mm thickinner wall layer and a 0.6 mm thick outer wall layer were produced. Tensingle-wall crucibles (Comparative Example 7) each having a 1 mm thicksingle wall layer were produced. Each of the thus produced crucibles wassubjected to a lifetime test which was conducted by melting 20 cc ofaluminum therein and then cooling the same. All ten crucibles of Example6 showed lifetimes of more than 8 cycles. In contrast thereto, sixcrucibles of Compartive Example 6 were broken by 3 cycles, and allcrucibles of Comparative Example 6 had been broken at the seventh cycle.Nine crucibles of Comparative Example 7 were broken at the first cycle,and the remaining one crucible was broken at the second cycle.

Examples 7 to 11 and Comparative Examples 8 to 12

Using eight graphite plates each having the dimensions of 50 cm(width)×60 cm (length)×1 cm (thickness), a reaction chamber having anoctagonal cross section was formed on a graphite plate (bottom plate)having a diameter of 20 cm. A 5 cm diameter hole was provided at thecenter of the bottom plate for introducing therethrough gases. Twographite pipes, one having an outer diameter of 5 cm and the otherhaving an outer diameter of 2.5 cm, coated preliminarily with pyrolyticboron nitride were connected coaxially to the hole of the bottom plate.A graphite substrate having a diameter of 5 cm and a length of 6 cm wassuspended in the upper portion of the reaction chamber. The reactionchamber was put into a vacuum furnace heated to a high temperature byresistance heating, and the graphite inner and outer pipes of thestarting gas inlet conduit were connected with stainless steel gasconduits for feeding, respectively, a BCl₃ gas and an NH₃ gas.

The furnace was evacuated to an order of 10⁻³ Torr and heated to 1900°C., and then the pyrolytic boron nitride crucibles each having a totalwall thickness of 1 mm were produced at a pressure of 1 Torr and underthe conditions as set forth in Table 2. The deposition was changed byvarying the molar ratio of the starting gases. Each of the thus producedcrucibles was subjected to the following lifetime test which wasconducted to simulate the operations for growing a single crystal by theLiquid Encapsulated Czochralski method, similarly as in Examples 1 to 5.

As will be apparent from the results shown in Table 2, the crucibles ofthe invention had considerably longer lifetimes as compared with thoseof the Comparative Examples such that the lifetimes of the crucibles ofthe invention were more than 20 cycles.

                                      TABLE 2                                     __________________________________________________________________________    First Wall Layer   Second Wall Layer                                                Molar Ratio                                                                          Thickness                                                                           Molar Ratio                                                                          Thickness                                                                           Lifetime of                                   Run No.                                                                             of NH.sub.3 /BCl.sub.3                                                               (micron)                                                                            of NH.sub.3 BCl.sub.3                                                                (micron)                                                                            Crucible                                      __________________________________________________________________________    Example 7                                                                           2      25    1      5     30 cycles                                     Example 8                                                                           10     25    1      5     32 cycles                                     Example 9                                                                           3.5    100   1/3    2     24 cycles                                     Example 10                                                                          3.5    20    1.8    5     38 cycles                                     Example 11                                                                          5      50    1      5     40 cycles                                     Com. Ex. 8                                                                          3.5    18    2.5    5     Deep cracking formed                                                          at 12th cycle                                 Com. Ex. 9                                                                          3.5    25    2.5    0.4   Deep cracking formed                                                          at 13th cycle                                 Com. Ex. 10                                                                         3.5    110   1/3    10    Delaminated at the                                                            initial stage                                 Com. Ex. 11                                                                         3.5    50    1.5    30    Deep cracking formed                                                          at 12th cycle                                 Com. Ex. 12                                                                         Commercially Available Crucible                                                                         15 cycles                                           (Single Wall Structure)                                                 __________________________________________________________________________

Example 12 and Comparative Examples 13 and 14

Generally following the procedures as described in Example 7, tenpyrolytic boron nitride crucibles (Example 12) each having a diameter of3.49 cm and a height of 4.13 cm were produced. Ten double-wall crucibles(Comparative Example 13) each having a 0.4 mm thick inner wall layer anda 0.6 mm thick outer wall layer and ten single-wall crucibles(Comparative Example 14) each having a 1 mm thick single wall layer wereproduced. Each of the thus produced crucibles was subjected to alifetime test which was conducted by melting 20 cc of aluminum thereinand then cooling the same. All ten crucibles of Example 12 had lifetimesof more than 8 cycles. In contrast thereto, six crucibles of ComparativeExample 13 were broken by 3 cycles, and all crucibles of ComparativeExample 13 had been broken at the seventh cycle. Nine crucibles ofComparative Example 14 were broken at the first cycle, and the remainingone crucible was broken at the second cycle.

Although the present invention has been described with reference to thespecific examples, it should be understood that various modificationsand variations can be easily made by those skilled in the art withoutdeparting from the spirit of the invention. Accordingly, the foregoingdisclosure should be interpreted as illustrative only and not to beinterpreted in a limiting sense. The present invention is limited onlyby the scope of the following claims.

What is claimed is:
 1. A pyrolytic boron nitride crucible having amulti-walled structure comprising at least five pyrolytic boron nitridewall layers, the thickness of each of first pyrolytic boron nitride walllayers being 5 to 100 microns, the thickness of each second pyrolyticboron nitride wall layers being 1/1 of that of the first wall layer, thetotal thickness of the entire wall layers being 0.5 to 3 mm, and saidfirst wall layer being bonded to and laminated alternately with saidsecond wall layer, whereby the peeling of one wall layer is preventedfrom affecting the other wall layers so that the peeling of one walllayer is not propagated into the adjacent layer.
 2. The pyrolytic boronnitride crucible according to claim 1, wherein the thickness of each ofsaid wall layers is 20 to 100 microns.